The present invention relates to an apparatus for correcting a recording density in a heat sensitizing recorder or a thermo-transfer recorder.
In the heat sensitizing recorder or the thermo-transfer recorder for performing a recording operation in white and black by using a thermal head having a plurality of heating resistors, a recording density correcting apparatus is constructed such that the power supplied to the heating resistors is constantly controlled to hold the recording density constantly by counting the number of black dots to be recorded and changing the width of a pulse applied to the heating resistors from this counted valve so as to control a heating amount of the heating resistors. Such a recording density correcting apparatus is discussed in Japanese Laid-Open Publication No. 58-124679 for example.
In the conventional recording density correcting apparatus, the number of black dots is counted and the width of the pulse applied to the heating resistors is changed from this counted value, thereby controlling the heating amount of the heating resistors. Accordingly, the heating amount of the heating resistors is controlled by a binary system by using information as to whether there is a black dot or not, so that such a correcting apparatus is applied to only a recorder for performing the recording operation in black and white. Therefore, the above recording density correcting apparatus cannot be applied to the recorder for recording an image at a multiple gradation and the power supplied to the heating resistors is changed by the number of operations of the plurality of heating resistors in the thermal head, thereby changing the recording density.
Therefore, in the recorder for recording an image at a multiple gradation, it is considered that the number of pulses applied to the plurality of heating resistors in the thermal head is counted every gradation level and the applied pulse width or the amplitude of applied pulse of the heating resistors is changed by each counted value every gradation level such that the recording density becomes constant. However, in such an apparatus, the construction of the apparatus is complicated and it is necessary to perform the processings at high speed.
Further, there is normally dispersion in resistance value of the heating resistors between the thermal heads, which dispersion occurs when they are manufactured. The heating amount of the heating resistors is changed by this dispersion and the recording density is thereby changed. The dispersion in average resistance value of the heating resistors is .+-.20% even in the thermal head of a thin film type in which the dispersion in resistance value of the heating resistors is relatively small. Accordingly, in the thermal head having the average resistance value 2000.OMEGA. of the heating resistors, there is dispersion ranged from minimum value 1600.OMEGA. to maximum value 2400.OMEGA. with respect to the average resistance value of the heating resistors and therefore the dispersion range is 800.OMEGA.. Therefore, when the energy applied to the heating resistors is corrected by correction data according to the dispersion in resistance value of the heating resistors with the reference of the resistance value thereof as 2000.OMEGA. such that the recording density becomes constant, energy E.sub.max applied to the heating resistors in maximum value R.sub.max of the average resistance value of the heating resistors is provided by the following formula, EQU E.sub.max =V.sup.2 t/R.sub.max =V.sup.2 t/1600
where the width of pulses applied to the heating resistors is t and the voltage applied to the heating resistors is V. Futher, when the average resistance value of the heating resistors in minimum value R.sub.min, energy E.sub.min applied to the heating resistors is provided by the following formula, EQU E.sub.min =V.sup.2 t/R.sub.min =V.sup.2 t/2400. EQU Accordingly, EQU E.sub.max /E.sub.min =2400/1600=1.5
so that the energy applied to the heating resistors is changed 1.5 times by the dispersion in resistance value thereof and it is therefore necessary and complicated to make the above correction data every time when the thermal heads are exchanged.